1. Field of the Invention
This invention relates to an input signal determining method and an apparatus for practicing the method, and more particularly to an input signal determining method and an apparatus for practicing the method which are applied to an electronic unit for a motor vehicle.
2. Description of the Related Art
In order to eliminate interference signals such as bound noises due to the chattering or vibration of switches, and external noises, a conventional electronic unit on a motor vehicle employs an apparatus based on a particular input signal determining method. The conventional apparatus of this type is as shown in FIG. 6.
As shown in FIG. 6, an input signal determining apparatus 30 comprises three series-connected first, second and third D-type flip-flop circuits 31, 32 and 33, an OR element, and three NAND elements. An input signal is applied through a terminal 35 to the D terminal of the first D-type flip-flop circuit 31, and its logic level is provided at the Q terminal. The logic level is applied to the D terminal of the second D-type flip-flop circuit 32. An output provided at the Q terminal of the D-type flip-flop circuit 32 is applied to the D terminal of the third D-type flip-flop circuit 33. A synchronizing signal is applied through a terminal 34 to those three D-type flip-flop circuits 31, 32 and 33. A reset signal is applied through a terminal 36 to the reset terminals of the D-type flip-flop circuits 31, 32 and 33.
The input signal is determined or judged based on the logic level of the input signal and the logic levels provided at the Q terminals of the three flip-flop circuits 31, 32 and 33, and the result of the determination is obtained through a terminal 37.
However, in order for the above-described input signal determining method to eliminate the misjudgment of the input signal, it is necessary to employ a number of logic circuits, which are rather troublesome.
In addition, the input signal determining apparatus, employing a number of logic circuits, is expensive. Besides, it is impossible for the apparatus to effectively eliminate synchronous noises.